Digital transmission systems send signals as a series of ones and zeros. In an optical transmission system, such as a fiber optic transmission system, the ones and zeros are represented by the presence or absence of an optical pulse. A number of different digital signals can be combined together with each signal occupying its own time slot in a digital pulse stream of higher rate than of each individual signal. This is known as "time-division multiplexing" (TDM) and it allows the optical digital signals to timeshare the same transmission line, such as an optical fiber. As used herein, the term multiplexed means time division multiplexed. At the receiving end of the optical transmission system the multiplexed signals are separated out (demultiplexed) to extract the individual signals, and the individual signals may be processed accordingly. It is noted that multiplexed signals can in turn be multiplexed together to form a hierarchy of multiplexed digital signals. In such a case, the demultiplexing may take place in several stages.
In conventional system, a multiplexed optical signal is first detected by electronic components, such as photodetectors, to convert the string of optical pulses into electrical pulses which contain the multiplexed signal. The demultiplexing is then performed electronically on the resulting electrical signal. Current limitations in the speed of the electronics limits the pulse rate of high speed transmission systems to about 10 Gbit/s. Current research is being done with a view toward increasing this speed to 20 Gbit/s. However, future optical transmission systems envision rates of 100 Gbit/s or higher which would be accomplished by wavelength division multiplexing or time division multiplexing.
A problem results in that demultiplexing an optical signal of 100 Gbit/s or greater clearly exceeds the limitations set by the speed of the detection electronics. As a result, techniques for performing the demultiplexing step optically, before the optical signal is converted into an electronic signal, have been investigated.
The techniques for optical demultiplexing typically involve nonlinear optical processes. A nonlinear optical demultiplexer generally requires: 1) a sequence of short optical pulses which are transmitted at the rate of the pulse sequence to be extracted from the multiplexed optical signal; 2) a means for synchronizing the series of pulses with the pulse sequence to be demultiplexed; and 3) a nonlinear medium which is usually an optical fiber but could also be a semiconductor amplifier. For further information on nonlinear devices see, M. N. Islam, Ultrafast Fiber Switching Devices and Systems, Cambridge Univ. Press (1992), which is incorporated herein by reference.
The fiber nonlinear optical devices which have been used or proposed for optical time-division demultiplexing are all based on the nonlinear refractive index of the optical signal path which is the small increase in the index of refraction with optical power. These devices are described below.
1. Kerr Effect Devices.
A Kerr effect device uses the nonlinear index to change the state of polarization out of a fiber. In a Kerr demultiplexer (shown in FIG. 1) a strong optical pulse which functions as a demultiplexing control pulse is transmitted over optical signal path 100 at wavelength.lambda..sub.p . A weaker multiplexed signal is transmitted over optical signal path 120 at wavelength .lambda..sub.s . The signals are combined by wavelength multiplexing coupler 110. For the portions of the multiplexed signal which are synchronized with the control pulse, birefringence from the control pulse changes the polarization of the weaker multiplexed signal transmitted in optical signal path 115. Bandpass filter 120 is used to filter out the optical pulse signal at wavelength.lambda..sub.p to allow only the demultiplexed signal at wavelength .lambda..sub.s to pass through to signal path 122. A polarizer 130 is provided so an output pulse on optical signal path 140 appears only when the desired multiplexed signal pulses are present. Polarization controllers (PC) 150, 152, 154 are required to ensure proper orientation of the states of polarization of the signal on optical signal path 120, the strong pulse on optical signal path 100, and the signal on optical signal path 122. About 1 kW peak power in 1 m of fiber (or 1 W in 1 km) is necessary to operate a Kerr device. For further information on Kerr effect multiplexing see, T. Morioka, M. Saruwatari, and H. Takara, Ultrafast Optical Multi/Demultiplexer Utilizing Optical Kerr Effect in Polarization-Maintaining Single-Mode Fibres, Electronics Letters Vol. 23, No. 9, pages 453-454, Apr. 23, 1987; and O. Duhr, Frank Seifert, and Valentin Petrov, Ultrafast Kerr Demultiplexing up to 460 Gbits/s in Short Optical Fibers, Applied Optics, Vol. 34, No. 24, Aug. 20, 1995, both of which are incorporated herein by reference.
2. Nonlinear directional coupler.
The basic nonlinear directional coupler (shown in FIG. 2) is a twin-core fiber or parallel planar guides. An intensity-dependent change in refractive index blocks the normal power exchange between the guides so the coupler 220 becomes an intensity-dependent optical switch. For demultiplexing, a length is chosen so that the multiplexed optical signal on path 200 appears on the optical signal path 200 after the coupler 220 only when the demultiplexing pulse is present on optical signal path 210. Beat lengths are short (1 meter would be long for a twin-core fiber) so the nonlinear coupler is a high power device. The power-length product is about the same as for a Kerr device. A nonlinear coupler many beat periods long is similar to a Kerr device. It also suffers similar requirements for active stabilization because of sensitivity to temperature and pressure fluctuations.
The basic principal of the nonlinear directional coupler is that some sort of coupling or transfer of light is blocked by an optical beam intense enough to change the refractive index. There is a family of such devices which appear to be quite different from the twin-core directional coupler but are similar both in function and optical power requirements. These devices are made with a two-mode fiber, a birefringent fiber, or a so-called `rocking filter fiber`. Mode beating in a two-mode fiber is just one limit of coupling in a twin-core fiber. In a birefringent fiber the sign of circular polarization reverses at half the birefringence beat length. High power changes the properties of the fiber and the input polarization propagates unchanged. A rocking filter coupler uses linear rather than circular polarizations in a birefringent fiber with periodic polarization coupling matched to the beat length. High optical power shifts the coupling wavelength. For further information on a rocking filter coupler see, C. G. Krautschik, P. Wigley, G. I. Stegeman, and R. H. Stolen, Demonstration of demultiplexing with a Rocking Filter Fiber, Appl. Phys. Lett., Vol. 63, No. 7, Aug. 16, 1993, pages 860-862, which is incorporated herein by reference.
3. Nonlinear Loop Mirror.
A fiber Mach-Zender interferometer is, in principle, a simple way to make a demultiplexer. Demultiplexing high-power pulses in one arm of the interferometer change the relative phase of the two signal paths meeting at the output coupler but it is difficult to achieve temperature stability in two long separate fibers. A nonlinear interferometer that does not require stabilization is a loop mirror made with a 3 dB fiber coupler and a loop of fiber as illustrated in FIG. 3. The loop mirror 310 is a perfect reflector and transmits nothing from its output port 340. A loop mirror is stable to environmental changes because the two arms of the interferometer are clockwise and counterclockwise paths in the same fiber so a very long fiber can be used. Demultiplexing pulses transmitted over optical signal path 320 copropagate with the signal pulses transmitted over optical signal path 330 in only one direction. By way of the nonlinear index, clockwise and counterclockwise signals experience a relative phase shift and signal power will be transmitted from the output port 340. Demultiplexing with 150 mW pulses was demonstrated in a 5 km loop in, B. P. Nelson, K. J. Blow, P. D. Constantine, N. J. Doran, J. K. Lucek, I. W. Marshall, K. Smith, All Optical Gbit/s Switching Using Nonlinear Optical Loop Mirror, Electronics Letters, Vol. 27, No. 9, Apr. 25, 1991, which is incorporated herein by reference. Nonlinear loop mirrors do require polarization control to ensure the proper state of polarization at the fiber coupler. Active stabilization is necessary because the state of polarization can vary with changes in temperature and pressure.
4. Four-wave mixing.
A four-wave mixing demultiplexer (shown in FIG. 4) utilizes mixing of waves through the nonlinear index. A pump signal transmitted over optical signal path 410 at frequency .omega..sub.p and a weaker multiplexed signal transmitted over optical signal path 420 at frequency .omega..sub.s generate an idler on signal path 430 at frequency .omega..sub.i =2.omega..sub.p -.omega..sub.s. The demultiplexed pulse stream transmitted over optical signal path 430 at frequency .omega..sub.i is isolated with a bandpass filter 440. If the pump wavelength is close to the zero-dispersion wavelength, the coherence length for four-wave mixing becomes extremely long. With 100 mW of pump power, 10 km of fiber, and wavelength separations 10 nm or less conversion efficiency from input signal could be 20 dB. For further information on four-wave mixing demultiplexers see, P. O. Hedekvist and P. A. Andrekson, Demonstration of Fibre Four-Wave Mixing Optical Demultiplexing with 19 dB Parametric Amplification, Electronics Letters, Vol. 32, No. 9, pages 830-831, Apr. 25, 1996, which is incorporated herein by reference.
All of these nonlinear fiber demultiplexers suffer from the choice between short fibers requiring high optical powers or long fibers with polarization control and some sort of active stabilization. As a general rule, the power-length product for all these nonlinear index devices is about 1 kW-m using standard fiber. Thus, a nonlinear fiber demultiplexer which would circumvent the tradeoff between length and stability is desirable.